A study using new imaging technology has found 'silent' heart attacks may be far more common, and more deadly, than suspected, say US researchers.

Some studies estimate that these often painless heart attacks, also known as unrecognised myocardial infarctions, affect 200,000 people in the United States and 300,000 in Europe each year.

But Dr Han Kim of Duke University in North Carolina suspects the numbers may be far higher.

"No one has fully understood how often these heart attacks occur and what they mean, in terms of prognosis," says Kim, whose study will appear in the Public Library of Science journal PLoS Medicine.

Doctors usually can tell whether a patient has had a recent heart attack by looking for signature changes on a test of the heart's electrical activity called an electrocardiogram, and by checking for certain enzymes in the blood.

For a heart attack that might have occurred in the past, doctors look for changes on an electrocardiogram called a Q-wave, a marker for damaged tissue.

But not all silent heart attacks result in Q-waves.

"Those are the ones we haven't been able to count because we've never had a good way to document them," Kim said.

To spot these, Kim and colleagues used a new type of magnetic resonance imaging technology called delayed enhancement cardiovascular magnetic resonance, which is especially adept at finding damaged heart tissue.

They studied 185 patients with coronary artery disease but no record of heart attacks, who were scheduled to have a test to look for possible blockages in their heart arteries.

They found that 35% of the patients had evidence of a prior heart attack. And they found that these so-called non-Q-wave heart attacks were three times more common than silent heart attacks with Q-waves.

They also found that after two years of follow up, people who had suffered a silent, non-Q-wave heart attack had an 11-fold higher risk of death from any cause and a 17-fold higher risk of death due to heart problems, when compared to patients who did not have any heart damage.

Kim says currently people who have had silent heart attacks are treated like other patients with heart disease.

But given the findings, he says new studies should look at the best way to care for these patients.